$upd_sql = "UPDATE table SET [Field1]=?, [Field2]=?, [Field3]=? where Field0=?";
$ins_sql = "INSERT INTO table ( [Field1], [Field2], [Field3], [Field0] ) VALUES ( ?, ?, ?, ? )";

$sth = $dbh->prepare($exists ? $ins_sql : $upd_sql);
$sth->execute(
   'stringvalue',
   numericvalue,
   booleanvalue,
   index,
);